Combining two vectors element-by-element

Question

I have 2 vectors, such as these:

A <- c(1,2,NA,NA,NA,NA,7)
B <- c(NA,NA,3,4,NA,NA,7)

I would like to combine them so that the resulti

Answer 1

A <- c(1,2,NA,NA,NA,NA,7)
B <- c(NA,NA,3,4,NA,NA,7)
C <- rowMeans(cbind(A,B),na.rm=TRUE)
C[which(!is.na(A*B))]<- -1
#[1]   1   2   3   4 NaN NaN  -1

Benchmarks:

Unit: microseconds
          expr    min     lq median     uq     max
1 Roland(A, B) 17.863 19.095 19.710 20.019  68.985
2   Sven(A, B) 11.703 13.243 14.167 14.783 100.398